Patents and publications demonstrating that the addition of promoters to a base metal improves the quality of catalysts exist in large numbers. Such elements are added in different forms such as mineral salts or organometallic compounds. In general, more active or more selective catalysts are obtained which are sometimes also more stable than the corresponding monometallic catalyst. The manner in which such modifying agents are introduced is not unimportant as it dictates the properties of the catalyst to a great extent.
Thus catalyst formulations used in hydrocarbon conversion processes have been the subject of a very large number of studies. Supported metallic catalysts have been described, in particular in U.S. Pat. No. 3,998,900 and French patent FR-A-2 495 605. They contain a metallic phase based on platinum, modified by an additional metal M such as tin, supported on refractory inorganic oxides such as alumina. Metal M is advantageously introduced using an organometallic compound of that metal M. Such a method of introducing metal M has already been described in patents U.S. Pat. No. 3,531,543 and U.S. Pat. No. 4,548,918.
The processes cited above describe the production of a catalyst using at least one organometallic compound of metal M. Metal M is introduced in the form of at least one organometallic compound selected from the group formed by complexes, in particular carbonyl or polyketone complexes of metals M, and metal hydrocarbyls of metal M such as alkyls, cycloalkyls, aryls, metal alkylaryls and metal arylalkyls.
Introducing the additional element M in the form of an organometallic compound leads to more effective catalysts but necessitates the use of an organic solvent. The impregnating solvent described in U.S. Pat. No. 4,548,918 is selected from the group formed by oxygen-containing solvents containing 2 to 8 carbon atoms per molecule, paraffin, naphthene or aromatic hydrocarbons essentially containing 6 to 15 carbon atoms per molecule, and halogen-containing oxygen-containing organic compounds containing 1 to 15 carbon atoms per molecule. Such solvents can be used alone or mixed together.